The present invention relates to a light emitting semiconductor element for emitting green or blue light.
Optical information technology has lately developed very rapidly with the increased production of laser discs and/or laser printers. Those devices have a semiconductor laser which emits red light as a light source. In order to achieve the high capacity recording, high speed reading, and/or high speed printing, the light source must be not only of a high power, but also short in wavelength. Although a short wavelength semiconductor has been proposed, a semiconductor laser which emits green or blue light (wavelength is 0.3-0.6 micron) has not yet been obtained. Further, a blue light emitting diode is necessary for providing full color display using light emitting diodes, however, no such diode has been obtained.
As for a short wavelength light emitting semiconductor element, in particular one which emits green and/or blue light, because of restriction of the energy gap, not only the II-IV group compound semiconductor like ZnS, ZnSe, and ZnTe, but also I-III-VI.sub.2 group (like Cu(GaAl)(SSe).sub.2 et al) compound semiconductor called chalcopyrite type, and/or II-IV-V.sub.2 group ((CdZn)(GeSi)P.sub.2 et al) compound semiconductor is promising.
FIG. 1 shows the relationships between the energy gap (horizontal axis) and the lattice constant (vertical axis) of II-VI group compound semiconductor, I-III-VI.sub.2 group compound semiconductor, II-IV-V.sub.2 group compound semiconductor, and mixed crystals of the same.
It is clear from FIG. 1 that ZnSSeTe in II-VI group can not provide a crystal composition in which an active layer and a clad layer are lattice-matched to a substrate as the energy gap difference is higher than 0.2 eV. Further, in case of hetero junction of ZnSe-ZnTe or ZnS-ZnTe, the bottom of the conduction band of ZnTe is higher than the bottom of the conduction band of ZnS or ZnSe, as shown in the energy-band diagram of FIGS. 2A and 2B. Therefore, in case of ZnSSeTe group, an electron is not effectively confined in an active layer, and no double hetero structure which operates stably has been obtained. Further, the II-VI group compound semiconductor crystal has a strong ionization tendency, and therefore, the control of conductivity by adding an impurity is difficult. Therefore, a current injection type light emitting diode using p-n junction has not been developed.
Thus, I-III-VI.sub.2 group chalcopyrite type compound semiconductor, and II-IV-V.sub.2 group chalcopyrite type compound semiconductor are considered promising, instead of a prior II-VI group compound semiconductor.
However, the chalcopyrite type semiconductor, which has a high energy gap, has the disadvantage that controlling the conductivity by adding an impurity is difficult, and a p-n junction is difficult to produce. This is the same as is in the case of ZnSSeTe group. Further, when a double hetero junction structure is produced, the semiconductors of the same group can not provide the large difference of the energy gap between an active layer and a clad layer.
Further, if we try to produce a chalcopyrite type active layer and II-IV group clad layer, the energy gap difference is not sufficient, and/or ZnSSeTe group cannot provide a p-n junction as described above.
Accordingly, a light emitting semiconductor device having an active layer by I-III-VI.sub.2 group or II-IV-V.sub.2 group chalcopyrite type compound semiconductor has been very difficult to produce.
As described above, prior compound semiconductors such as II-VI group, I-III-VI.sub.2 group, or II-IV-V.sub.2 group chalcopyrite type used for providing an active layer for green or blue light can not provide a suitable clad layer because of a deficiency in conductivity control by impurity, sufficient energy gap difference between an active layer and a clad layer, and lattice matching to a substrate.